1. Field of the Invention
The present invention relates to a housing structure and a steering apparatus.
2. Description of the Related Art
An assist torque mechanism for a power steering apparatus is available which transmits a torque generated by a motor to a rack shaft via a belt transmission mechanism and a ball screw (see, for example, Japanese Patent Application Laid-open No. 2011-42268). The rack shaft, the belt transmission mechanism, and the ball screw are housed inside a housing divided into two parts in an extending direction of the rack shaft, in other words, a housing divided into two parts in a lateral direction.
The rack shaft is supported by each of the right and left divisional housings. Therefore, the housings need to be precisely combined together so as to accurately center the rack shaft with respect to both housings. Examples of conventional structures that allow the housings to be precisely combined together are depicted in FIG. 9A and FIG. 9B. In the structure depicted in FIG. 9A, over an inner fitting portion 83 formed at an opening edge of a housing 81, an outer fitting portion 84 formed at an opening edge of another housing 82 is externally fitted. A seal member 85 is interposed between the inner fitting portion 83 and the outer fitting portion 84 to prevent water from infiltrating into the housing.
In the structure depicted in FIG. 9B, a plurality of positioning pins 86 is interposed between a mating surface 87 of the housing 81 and a mating surface 88 of the housing 82. The seal member 85 is interposed between the mating surface 87 and the mating surface 88. This structure eliminates the need for a fitting structure for the inner fitting portion 83 and the outer fitting portion 84 in contrast to the structure in FIG. 9A, and allows the housing 81 and the housing 82 to be precisely combined together by bringing the mating surface 87 into abutting contact with the mating surface 88.
[Patent Literature 1] Japanese Patent Application Laid-open No. 2011-42268
However, the structure in FIG. 9A poses the following problems. The housings are shaped generally like an ellipse as viewed in the lateral direction, so as to conform to the arrangement of the belt transmission mechanism housed inside the housings. Therefore, the inner fitting portion 82 and the outer fitting portion 84 formed all along opening edges of the housings 81 and 82 are annularly formed so as to be generally elliptic. Thus, disadvantageously, time and effort are needed for operations to enhance precision, as represented by polishing of the fitting surfaces of the inner fitting portion 82 and the outer fitting portion 84.
The structure in FIG. 9B needs an operation of attaching the pins 86 to the housing 81 or the housing 82, which disadvantageously often needs time and effort. Moreover, pinholes in the housing 81 need to be precisely aligned with pin holes in the hosing 82.
Other conventional problems are as follows. In general, leg portions are formed on the housings 81 and 82 so as allow the housings 81 and 82 to be fixedly fastened to leg seats on a vehicle body. FIG. 10 depicts an example where a first leg portion 91 is formed on the housing 81 and where a second leg portion 92 is formed on the housing 82. In this regard, the two leg seats (not depicted in the drawings) on the vehicle body may be misaligned with each other due to, for example, a manufacture error or an assembly error. In this case, for example, when the first leg portion 91 is attached to one of the leg seats on the vehicle body, the second leg portion 92 may not be aligned with the other leg seat on the vehicle body. This may make attachment of the second leg portion 92 to the other leg portion difficult.